Achamana

There are three types of Āchamanam,[2] namely, Śrautācamanam (Sanskrit: श्रौताचमनम्), Smṛtyācamanam (Sanskrit: स्मृत्याचमनम्)[note 1] and Purāṇācamanam (Sanskrit: पुराणाचमनम्)[note 2].

ācamanam trividham - śrautaṃ smārtaṃ paurāṇaṃ ceti
tatra pratyakṣaśruticoditaṃ śrautam, sūtroktaṃ smārtam
keśavādyaistribhiḥ pītveti vacanoktaṃ paurāṇam[3]
Achamana is of three types - Śrautā, Smārta (as directed by Smritis) and Paurāṇā (as per Puranas).
There Śrautācamanam is directly instructed by Śruti (i.e. Vedas), Smārtācamanam is formulated by dharmasūtras (i.e. Smṛti).
Sippping (of water) thrice with the (24) names (of Vishnu) starting with keśava is called Paurāṇācamanam

However, in the Sandhyavandana ritual, there exists fourth version of āchamana, known as mantrācamana.

In śrautācamanam, water is sipped three times and is accompanied by the recitation of the three padas of the Gayatri in succession. Then, 21 parts of the body are touched while the 21 mantras are recited: the nine Abliṅgas, [note 3] the seven Vyahritis[note 4] (the names of the seven worlds, preceded by the sacred pranava Om), and the 3 padas of Gayatri siras. It is defined in ṣatkarma candrikā as[5]

devyāḥ pādaistribhiḥ pītvā abliṅgairnvabhiḥ spṛśet
saptavyāhṛtisaṃyuktā gāyatrī tripadā śiraḥ[note 5]
-ṣatkarma candrikā

The Smṛtyācamanam is an abridged version of the śrautācamanam. Water is sipped three times, accompanied by the ritual utterance svāhā. Then, 9 parts of the body are touched, accompanied by recitation of the specific sutras that instruct the version of the achamana in question.

Purāṇācamanam is done with the 24 names of Vishnu starting with keśava, etc.[note 6] The water is poured on the right hand palm, which made as gokurna sipped thrice with the first three names. Both hands are washed with the next two names.[note 7] For the remaining 19 names different parts of body are ritually cleansed.